This invention relates to the measurement of high temperatures and is particularly concerned with the measurement of the very high temperatures encountered in the core of a nuclear reactor. More particularly the present invention relates to the determination of temperatures employing a high temperature ultrasonic thermometer and is specifically directed to an improved design which prevents noise echoes from interfering with the signal echo of the ultrasonic thermometer.
The research and development as well as the operation of new energy sources require the determination of temperatures including extremely high temperatures which cannot be measured employing common thermometry techniques. In nuclear reactors, and particularly experimental reactors, it is extremely desirable and necessary to measure the temperature at various points within the reactor including various points within the reactor core. While thermocouples are commonly employed for the determination or the measurement of temperatures in many locations about and within the reactor, extremely high temperatures such as are reached in the nuclear reactor core preclude the use of thermocouples which are limited at very high temperatures by electrical shunting effects in the insulation material. Consequently, the extremely high temperatures reached in the core have been measured by the use of ultrasonic thermometers.
An ultrasonic thermometer utilizes the temperature dependent ultrasonic propagation velocity in a thin wire sensor as a temperature transducing mechanism. Since the propagation velocity is temperature dependent, the device is calibrated and the temperature is determined by measuring the propagation time between the signal echoes generated at discontinuities placed along the sensor and a refractory metal wire ultrasonic transmission line, the propagation time of the ultrasonic signal along the sensor and the transmission line being directly related to the temperature.
A high temperature ultrasonic thermometer has been employed in tests being cnducted in the Loss of Fluid Test (LOFT) and Power Burst Facility (PBF) reactors at the Idaho National Engineering Laboratory located at the National Reactor Testing Station in southeastern Idaho. These tests have included the measurement of the extremely high temperatures reached at the fuel rod centerlines. For these fuel rod centerline determinations, a specific high temperature ultrasonic thermometer was designed which included an ultrasonic transducer, a refractory metal wire ultrasonic transmission line with an ultrasonic sensor at the end of the transmission line and a protective sheath surrounding the transmission line and sensor. Since the length of the sensor is known and the propagation velocity is temperature dependent, the device can be calibrated for temperature as a function of propagation velocity. The reflections of the ultrasonic pulses that travel the known distance are used to determine the average propagation velocity in the sensor and hence determine the average temperature over the length of the sensor wire.
Ideally, the reflection of the ultrasonic waves occurs from the discontinuity where the sensor wire is attached to the lead-in transmission wire and from the end of the sensor wire. However, in actual practice it has been found that at the extremely high temperatures encountered in these measurements and particularly at temperatures above approximately 1600.degree. C., the transmission line often diffusion bonds or "sticks" to the protective sheath at points where they make contact. The "sticking" is very troublesome because it produces an acoustic impedance mismatch and correspondingly, an ultrasonic echo. This noise echo can interfere with the measurement of the propagation time between the sensor echoes. In fact, since an echo will be generated at any discontinuity along the transmission line, including those produced where the transmission line bonds or sticks to the protective sheath, noise echoes can be generated which at times can even mask out the signal echoes. Since tests are to be conducted at temperatures in excess of 2500.degree. C., a satisfactory solution to this bonding or sticking problem must be found.
A more complete description of the ultrasonic thermometer as well as the sticking problem is contained in U.S. AEC Report ANCR-1091, "High Temperature Ultrasonic Thermometer In-Reactor Fuel Rod Centerline Temperature Test Results," coauthored by the present applicant which report is incorporated herein by reference as though fully set forth.
One proposed solution to the sticking problem has been to use a spacer wire between the transmission line and the protective sheath. This spacer wire is helically wrapped about the transmission line. However, it has been found that the spacer wire sticks to the transmission line and protective sheath creating a line contact between the transmission line and the sheath which generates an exceptionally undesirable broad noise echo. Consequently, it is an object of the present invention to provide an improved design and a high temperature ultrasonic thermometer which will minimize the sticking of the transmission line to the sheath.
It is another object of the present invention to provide an improved design to prevent noise echoes from interfering with the desired signal echoes.
It is another object of the present invention to provide a method for minimizing the points of contact between the transmission line and the protective sheath.
Other objects and advantages of the present invention will become apparent upon reading the following description and with particular reference to the specific embodiment described hereinbelow.